Two aspects of the platelet contractile system will be studied: 1. The structure of contractile system components, and in particular troponin, at the molecular level.2. The machanics of platelet contraction and relaxation studied in an isometric system that we already described (Nature, 246, 36, 1973). The study of kinetics of platelet contraction reflects the interaction among the various components of the contractile system, and therefore may be of important value for the elucidation of the platelet contractile mechanism.